After a 22-year absence the critically endangered stonefly Isogenus nubecula has been rediscovered in the River Dee in North Wales by John Davy-Bowker of the Freshwater Biological Association. Part of an ancient group of insects that have changed little since the Permian period 250 million years ago, stoneflies are typically found in cool, well oxygenated waters and are particularly susceptible to pollution. Generally taking a year to mature, they grow as nymphs clinging under stones in fast flowing areas of streams and rivers, in the spring emerging into winged adults which mate and lay the next generation of eggs.

Isogenus nubecula has been progressively lost from most of Western Europe. Whilst still found in Sweden, Eastern Europe and further east, its West European distribution has been confined to the River Dee in Wales. Even here its distribution had been shrinking and by the early 1990’s its presence was confined to just a handful of sites. In 1995 during routine monitoring by the Environment Agency (now Natural Resources Wales) just a single individual was found. This was the last recorded presence of the species in Western Europe.

Despite numerous return surveys the species could not be found. In a recent review by Craig Macadam, Conservation Director at Buglife, Isogenus was assigned the international threat status ‘Critically Endangered’ to reflect its rarity and decline.

Despite its demise, not everyone gave up on this iconic Riverfly, and in early March this year John Davy-Bowker, a freshwater biologist with Freshwater Biological Association revisited its former site once again. After several hours of diligent hunting John was delighted to find Isogenus nubecula nymphs, albeit in low numbers, but alive and well in the Welsh River Dee once more.

Samples have now been returned to the FBA River Laboratory in Dorset and a follow up survey has been carried out by colleague Mike Hammett from Anglesey. Both John and Mike are rearing adults from the nymphs they have collected and are gathering as much information as they can to understand what makes this iconic species so rare.